Production of 2-chloro-2-hydrohexafluoropropane and azeotropes thereof with HF

ABSTRACT

A process is disclosed for the monohydrogenolysis of 2,2-dichlorohexafluoropropane to 2-chloro-2-hydrohexafluoropropane. The process involves reacting the 2,2-dichlorohexafluoropropane with hydrogen at an elevated temperature of about 150° C. or less in the presence of a catalyst containing a catalytically effective amount of palladium supported on trivalent chromium oxide in the presence of an acid of the formula HZ (where Z is Cl and/or F) to produce 2-chloro-2-hydrohexafluoropropane with a selectivity of over 70% based upon the 2,2-dichlorohexafluoropropane converted. Azeotropes of 2-chloro-2-hydrohexafluoropropane with HF are also disclosed; as are processes for producing such azeotropes.

RELATED APPLICATION

This application is national filing under 35 USC 37 of InternationalApplication No. PCT/US95/15625, which is a continuation-in-part ofpending U.S. patent application Ser. No. 08/351,927 filed Dec. 8, 1994,which issued as U.S. Pat. No. 5,481,051.

FIELD OF THE INVENTION

This invention relates to the production of hydrohalofluorocarbons andtheir azeotropes with HF, and more particularly to the hydrogenolysisreactions of 2,2-dichlorohexafluoropropane using palladium-containingcatalysts and products which can be produced thereby.

BACKGROUND

Various processes for the catalytic hydrogenolysis ofchlorofluorocarbons and hydrochlorofluorocarbons are known. For example,U.S. Pat. No. 2,942,036 discloses the reaction of1,2,2-trichloropentafluoropropane with hydrogen in the presence ofpalladium on activated carbon catalyst to produce1,2,2-trihydropentafluoropropane. The examples show that under theconditions of the experiments one of the products from this reaction isCF₃ CH═CF₂. Japanese Patent Application Publication Hei 1(1989)-319441discloses a process where one chlorine atom is selectively replaced byhydrogen in 1,1,1-trichlorotrifluoroethane using a platinum catalyst.For comparison, a palladium on carbon catalyst is disclosed to produce1,1,1-trifluoroethane as the major product under the conditions of theexperiment.

It is well known that the hydrogenolysis of compounds such aschlorofluorocarbons to replace chlorine by hydrogen produces hydrogenchloride as a co-product. Loss of fluorine when it occurs (e.g., toproduce overhydrogenated products) can produce HF as a by-product.

SUMMARY OF THE INVENTION

The present invention provides a process for the monohydrogenolysis of2,2-dichlorohexafluoropropane (i.e., CF₃ CCl₂ CF₃, or CFC-216aa) to2-chloro-2-hydrohexafluoropropane (i.e., CF₃ CHClCF₃, or HCFC-226da).The process comprises reacting said 2,2-dichlorohexafluoropropane withhydrogen at an elevated temperature of about 150° C. or less in thepresence of a catalyst containing a catalytically effective amount ofpalladium supported on trivalent chromium oxide in the presence of anacid of the formula HZ, where Z is selected from the group consisting ofCl, F and mixtures thereof, to produce 2-chloro-2-hydrohexafluoropropanewith a selectivity of over 70% based upon the2,2-dichlorohexafluoropropane converted.

Azeotropic compositions (e.g., an azeotropic composition comprising fromabout 80 to 47 mole percent HF and from about 20 to 53 mole percent CF₃CHClCF₃) are also provided; as is a process for producing an azeotropiccomposition of HF and 2-chloro-2-hydrohexafluoropropane as a product ofthe monohydrogenolysis of 2,2-dichlorohexafluoropropane where HF ispresent.

DETAILED DESCRIPTION

The catalysts suitable for the process of this invention comprisepalladium and may optionally contain other components such as otherGroup VIII metals. The palladium is supported on chromium oxide. Anysource of chromium oxide is suitable, but chromium oxide prepared by thethermal decomposition of (NH₄)₂ Cr₂ O₇ is especially preferred. Aprocedure for the preparation of Cr₂ O₃ by the thermal decomposition of(NH₄)₂ Cr₂ O₇ is disclosed in U.S. Pat. No. 5,036,036, the entirecontents of which are incorporated herein by reference.

The acid HZ is at least partially produced during the reaction as thehalogen Cl is removed from the starting material as a result of thehydrogenolysis. Accordingly, Z is ordinarily at least in part Cl. Alsoof note are embodiments where Z is partially F (i.e., the acid is amixture of HCl and HF). HF can be present for example, as a result ofoverhydrogenolysis, wherein fluorine substituents of the startingmaterial are partially replaced by hydrogen. HF can also be present inthe reaction feed. For example, residual HF can be present fromprocesses used to make the 2,2-dichlorohexafluoropropane. Of note inthis regard are embodiments where said starting material is a componentof an azeotrope of HF and said starting material, and starting materialfrom said azeotrope is reacted with hydrogen in the presence of HF fromsaid azeotrope.

Unlike alumina supports which are readily fluorinated, chromiafluorinates much more slowly under the same reaction conditions. Withoutwishing to be bound by theory, it is believed that because of the slowerfluorination, chromia supports maintain their surface area longer thanalumina supports; thereby enhancing catalyst life.

The palladium-containing material used to prepare the catalyst ispreferably from a palladium salt (e.g., palladium chloride). The othermetals which may be added to the catalyst include those from Group VIII(e.g., Pt, Rh, Ru or Ni). The metal may be added in the conventionalmanner (e.g., as a soluble salt of the metal). The concentration ofpalladium supported on the chromium oxide support is typically withinthe range from about 0.2% to about 5% by weight of the catalyst. Theconcentration of other Group VIII metals, when present, is typicallywithin the range of from 0% to about 3% by weight of the catalyst, butpalladium is ordinarily at least 60% by weight of the total supportedmetal, (preferably, at least 80% of the total supported metal).

The hydrogenolysis of the present invention is conducted at an elevatedtemperature. Ordinarily the temperature is about 150° C. or less.Typically satisfactory reaction rates are achieved at operatingtemperatures of about 100 to 125° C. Generally, in order to providesubstantial hydrogenolysis product yields, the amount of hydrogen usedis at least about 0.5 mole per mole of the organic starting material. Toprovide yields desired in many embodiments, at least stoichiometricamounts of hydrogen are used. A considerable excess of hydrogen can alsobe advantageously employed to provide the yields desired in manyembodiments in addition to serving as a heat sink to reduce the overalltemperature rise in the reactor. The amount of the monohydrogenolysisproduct in the reaction product mixture containing the same number offluorines as the starting material is typically at least 70%.

The process of this invention is especially suitable for the productionof 2-chloro-2-hydrohexafluoropropane (HCFC-226da) from2,2-dichlorohexafluoropropane (CFC-216aa). The monohydrogenolysisproduct, HCFC-226da is a valuable intermediate for the synthesis ofother fluorine containing materials, such as CF₃ CHFCF₃ (HFC-227ea)which is useful as a fire extinguishant.

Pressure is not critical. Atmospheric and superatmospheric pressures arethe most convenient and are therefore preferred.

The reaction products may be separated by conventional techniques, suchas distillation. Hydrochlorofluorocarbons such as2-chloro-2-hydrohexafluoropropane (HCFC-226da) likely form azeotropeswith HF; and conventional decantation/distillation may be employed iffurther purification of HCFC-226da is desired. An azeotrope is a liquidmixture that exhibits a maximum or minimum boiling point relative to theboiling points of surrounding mixture compositions. A characteristic ofminimum boiling azeotropes is that the bulk liquid composition is thesame as the vapor compositions in equilibrium therewith, anddistillation is ineffective as a separation technique. It has beenfound, for example, that CF₃ CHClCF₃ (HCFC-226da) and HF form a minimumboiling azeotrope. This azeotrope can be produced as a co-product withHCFC-226da. As discussed further below, compositions may be formed whichconsist essentially of azeotropic combinations of hydrogen fluoride withHCFC-226da. These include a composition consisting essentially of fromabout 47 to 80 mole percent HF and from about 53 to 20 mole percentHCFC-226da (which forms an azeotrope boiling at a temperature between-50° C. and about 130° C. at a pressure between about 7.2 kPa and about4391 kPa). The hydrochlorofluorocarbons (e.g., HCFC-226da) can beseparated from the HF in such azeotropes by conventional means such asneutralization and decantation. However, azeotropic compositions of thehydrochlorofluorocarbons and HF (e.g., an azeotrope recovered bydistillation of hydrogenolysis reactor effluent) are useful as recycleto a fluorination reactor, where the recycled HF and the recycledhydrochlorofluorocarbon can function as reactants. Thus, for example,the process of this invention for producing CF₃ CHClCF₃ by the reactionof CF₃ CC₁₂ CF₃ with H₂ in the presence of HF can further comprise thesteps of recovering a portion of the CF₃ CClHCF₃ as an azeotropiccomposition of CF₃ CClHCF₃ and HF; and can be followed by a process forproducing CF₃ CHFCF₃ which comprises recycling said azeotropiccomposition to a fluorination reactor where CF₃ CClHCF₃ is reacted withHF. CF₃ CHFCF₃ is useful as a fire extinguishant.

HCFC-226da/HF Azeotrope

As noted above, the present invention provides a composition whichconsists essentially of hydrogen fluoride and an effective amount of aCF₃ CHClCF₃ to form an azeotropic combination with hydrogen fluoride. Byeffective amount is meant an amount which, when combined with HF,results in the formation of an azeotrope or azeotrope-like mixture. Asrecognized in the art, an azeotrope or an azeotrope-like composition isan admixture of two or more different components which, when in liquidform under given pressure, will boil at a substantially constanttemperature, which temperature ay be higher or lower than the boilingtemperatures of the individual components, and which will provide avapor composition essentially identical to the liquid compositionundergoing boiling.

For the purpose of this discussion, azeotrope-like composition means acomposition which behaves like an azeotrope (i.e., has constant-boilingcharacteristics or a tendency not to fractionate upon boiling orevaporation). Thus, the composition of the vapor formed during boilingor evaporation of such compositions is the same as or substantially thesame as the original liquid composition. Hence, during boiling orevaporation, the liquid composition, if it changes at all, changes onlyto a minimal or negligible extent. This is to be contrasted withnon-azeotrope-like compositions in which during boiling or evaporation,the liquid composition changes to a substantial degree.

Accordingly, the essential features of an azeotrope or an azeotrope-likecomposition are that at a given pressure, the boiling point of theliquid composition is fixed and that the composition of the vapor abovethe boiling composition is essentially that of the boiling liquidcomposition (i.e., no fractionation of the components of the liquidcomposition takes place). It is also recognized in the art that both theboiling point and the weight percentages of each component of theazeotropic composition may change when the azeotrope or azeotrope-likeliquid composition is subjected to boiling at different pressures. Thusan azeotrope or an azeotrope-like composition may be defined in terms ofthe unique relationship that exists among components or in terms of thecompositional ranges of the components or in terms of exact weightpercentages of each component of the composition characterized by afixed boiling point at a specified pressure. It is also recognized inthe art that various azeotropic compositions (including their boilingpoints at particular pressures) may be calculated (see, e.g., W.Schotte, Ind. Eng. Chem. Process Des. Dev. 1980, 19, pp 432-439).Experimental identification of azeotropic compositions involving thesame components may be used to confirm the accuracy of such calculationsand/or to modify the calculations for azeotropic compositions at thesame or other temperatures and pressures.

It has been found that azeotropes of HF and HCFC-226da are formed at avariety of temperatures and pressures. At a pressure of 30.47 psia (210kPa) and 20° C., the azeotrope vapor composition was found to be about69.5 mole percent HF and about 30.5 mole percent HCFC-226da. Based uponthe above findings, it has been calculated that an azeotropiccomposition of about 79.7 mole percent HF and about 20.3 mole percentHCFC-226da can be formed at -50° C. and 1.04 psia (7.2 kPa) and anazeotropic composition of about 47.4 mole percent HF and about 52.6 molepercent HCFC-226da can be formed at 130° C. and 637 psia (4391 kPa).Accordingly, the present invention provides an azeotrope orazeotrope-like composition consisting essentially of from about 80 to 47mole percent HF and from about 20 to 53 mole percent HCFC-226da, saidcomposition having a boiling point from about -50° C. at 7.2 kPa toabout 130° C. at 4391 kPa.

Practice of the invention will become further apparent from thefollowing non-limiting examples.

EXAMPLE

CF₃ CCl₂ CF₃ →CF₃ CHClCF₃

Catalyst Preparation

A solution containing palladium chloride (2.88 g), conc. hydrochloricacid (3 mL) and deionized water (100 mL) was prepared in a round-bottomflask. To this solution was added chromium oxide, Cr₂ O₃, (98 g, 10×20mesh (1.7×0.83 mm)) prepared by the pyrolysis of (NH₄)₂ Cr₂ O₇. Theresulting slurry was stirred frequently and then dried in air at 150° C.for about 18 hours; followed by calcination in air for about 8 hours.Palladium on chromium oxide (96.7 g), containing about 2% palladium wasisolated.

Hydrogenolysis of CFC-216aa using Palladium on Chromium Oxide catalyst

Liquid CFC-216aa (CF₃ CCl₂ CF₃), 3 mL/hour was vaporized and mixed with20 cc/minute of hydrogen. This vapor mixture was sent through a 0.5"(1.3 mm) O.D.×8" (203 mm) Hastelloy™ nickel alloy reactor containing19.2 g of 10×20 mesh (1.7 mm×0.83 mm) palladium on chromium oxidecatalyst (2 weight percent palladium) heated in a fluidized sand bathmaintained at 100° C. The catalyst was heated at 400° C. in a stream ofhydrogen fluoride for about 30 minutes and subsequently reduced in astream of hydrogen at about 150° C. for about two hours prior to use (at100° C.) for the hydrogenolysis. Organic product analysis usingconventional gas chromatography after the catalyst was in use for abouttwenty hours of operation showed that CFC-216aa conversion was about92%. The hydrogen-containing products included 4.0% HFC-236fa (CF₃ CH₂CF₃), 86.0% HCFC-226da (CF₃ CHClCF₃) and small amounts of otherproducts. Only a small portion of the total reactor effluent was sent tothe gas chromatograph for organic product analysis. The bulk of theproduct stream which also contains inorganic acids such as HCl and HFwas sent to a caustic scrubber for neutralization of the acids.

The above reaction was repeated except that the reaction temperature was150° C. CFC-216aa conversion was essentially complete. Thehydrogen-containing products included about 9.5% HFC-236fa and 83%HCFC-226da and small amounts of other products.

The above reaction was repeated except that the reaction temperature was200° C. Again, CFC-216aa conversion was essentially complete. Inaddition to the hydrogen-containing products, HFC-236fa (23%) andHCFC-226da (62%), there was about 10% propane in addition to other minorby-products.

Comparative Example Hydrogenolysis of CFC-216aa

Using Palladium on Low-Ash Acid-Washed Carbon

Carbon Support

The carbon support used in the examples was a 4×8 mesh (about 4.7 mm×2.4mm) commercial grade coconut shell carbon which had (before washing) anash content of about 2.6 weight percent. After hydrochloric acidwashing, the carbon support had an ash content of less than about 0.1weight percent.

Liquid CFC-216aa, 3 mL/hour, was vaporized and mixed with 10 cc/minuteof hydrogen. This vapor mixture was sent through a 0.5" (12.7 mm)O.D.×8" (203 mm) Hastelloy™ nickel alloy reactor containing 7.2 g of 0.5weight percent palladium supported on acid-washed carbon maintained at150° C. using a fluidized sand bath. Only a small portion of the totalreactor effluent was sent to the gas chromatograph for organic productanalysis. The bulk of the product stream which also contains inorganicacids such as HCl and HF was sent to a caustic scrubber forneutralization of the acids. Organic product analysis using conventionalgas chromatography indicated that about 90% of the starting material hadbeen converted. The hydrogen-containing products included 15.7%2,2-dihydrohexafluoropropane (HFC-236fa), 54.3%2-chloro-2-hydrohexafluoropropane (HCFC-226da), 12.3%2-hydropentafluoropropene, and 1.7% 1,2,2-trihydropentafluoropropane(HFC-235fa) and small quantities of other compounds.

This example was repeated except that the hydrogen flowrate wasincreased to 30 cc/minute. Organic product analysis using conventionalgas chromatography indicated that the starting material conversion wasessentially complete. The hydrogen-containing products included 24.8%2,2-dihydrohexafluoropropane (HFC-236fa), 54.6%2-chloro-2-hydrohexafluoropropane (HCFC-226da) and 19.8%1,2,2-trihydropentafluoropropane (HFC-235fa) and small quantities ofother compounds.

This comparative experiment illustrates that when using palladiumsupported on acid-washed carbon as catalyst for the hydrogenolysis ofCFC-216aa (where two chlorines of the starting compound are on themiddle carbon and the two adjacent carbons contain trifluoromethylgroups) an olefin and/or a saturated product containing one lessfluorine than the starting compound can be produced in significantamounts.

What is claimed is:
 1. An azeotropic composition consisting essentiallyof from about 80 to 47 mole percent HF and from about 20 to 53 molepercent CF₃ CHClCF₃, which forms an azeotrope boiling at a temperaturebetween about -50° C. and 130° C. at a pressure between about 7.2 kPaand 4391 kPa.
 2. The azeotrope composition of claim 1 produced by aprocess comprising the steps of reacting 2,2-dichlorohexafluoropropanewith hydrogen at an elevated temperature of about 150° C. or less in thepresence of a catalyst containing a catalytically effective amount ofpalladium supported on trivalent chromium oxide in the presence of HF toproduce 2-chloro-2-hydrohexafluoropropane with a selectivity of over 70%based upon the 2,2-dichlorohexafluoropropane converted; and recovering aportion of the CF₃ CHClCF₃ as an azeotropic composition of CF₃ CHClCF₃and HF.
 3. The azeotropic composition of claim 1 which consistsessentially of about 69.5 mole percent HF and about 30.5 mole percentCF₃ CHClCF₃, which forms an azeotrope boiling at a temperature of about20° C. and a pressure of about 210 kPa.
 4. The azeotrope composition ofclaim 3 produced by a process comprising the steps of reacting2,2-dichlorohexafluoropropane with hydrogen at an elevated temperatureof about 150° C. or less in the presence of a catalyst containing acatalytically effective amount of palladium supported on trivalentchromium oxide in the presence of HF to produce2-chloro-2-hydrohexafluoropropane with a selectivity of over 70% basedupon the 2,2-dichlorohexafluoropropane converted; and recovering aportion of the CF₃ CHClCF₃ as an azeotropic composition of CF₃ CHClCF₃and HF.
 5. An azeotropic composition of 2-chloro-2hydrohexafluoropropaneand HF produced by a process comprising the steps of reacting2,2-dichlorohexafluoropropane with hydrogen at an elevated temperatureof about 150° C. or less in the presence of a catalyst containing acatalytically effective amount of palladium supported on trivalentchromium oxide in the presence of HF to produce2-chloro-2-hydrohexafluoropropane with a selectivity of over 70% basedupon the 2,2-dichlorohexafluoropropane converted; and recovering aportion of the CF₃ CHClCF₃ as an azeotropic composition of CF₃ CHClCF₃and HF.